The present invention relates generally to methods and compositions useful for binding magnetic materials to nonmagnetic surfaces. The products so formed are useful as magnetic recording media.
Magnetic recording media presently are used in a variety of forms, e.g., audiotape, videotape, flexible disks, hard disks, etc. Such recording media are comprised essentially of a magnetic recording layer provided on a nonmagnetic support. The magnetic recording layer is comprised of a ferromagnetic powder dispersed in a binder system. Usually, the ferromagnetic powder is a metal oxide having ferromagnetic properties. However, ferromagnetic metals and alloys may also be employed. (See, for example, U.S. Pat. No. 4,820,581) Additional components, such as lubricants, abrasives, and conductants may also be present in the medium.
The binder composition used to attach the ferromagnetic powder to the support is particularly problematic due to the several physical properties that must be optimized to produce a commercially feasible product. For example, the magnetic layer must be tough enough to withstand impacts but flexible enough to permit coiling, when the medium is tape, without breaking or cracking the recording medium. Also, the recording medium must exhibit good layer-to-layer adhesion to prevent peeling and spalling of the recording layer from the support. Additionally, the binder composition must afford good durability and stability to environmental factors, such as resistance to oxidation when the ferromagnetic material is a metal or alloy.
Previous methods for making magnetic recording media typically employ a binder system comprised of various amounts of polymer blends and additives. Additives such as multifunctional isocyanates and lubricants are exemplary. Polyblends or resins such as high molecular weight hydroxyl-containing resins are exemplary compounds for magnetic binders. See, for example, Chemical Encyclopedia, 14:732 (1985). The additives or resins are added to the magnetic layer composition in order to impart physical toughness to the layer, to improve the oxidative, thermal and hydrolytic stability of the layer, to enhance the dispersion of ferromagnetic particles, etc.
A common method for increasing the physical toughness of a magnetic coating is to perform crosslinking reactions of multifunctional components in the mixture. One type of crosslinking reaction employed in this regard is the stepwise reaction of high molecular weight hydroxyl functional species with multifunctional isocyanates. Usually the hydroxyl functional species is a long-chain polymeric species having a high molecular weight, e.g., greater than 20,000 D. However, it is generally found that high molecular weight hydroxyl-containing species afford inferior recording media because of the impact on dispersion quality, layer-to-layer adhesion, wound pile stability, and because of the large amounts required to achieve a high cross-link density.
Few examples of magnetic recording compositions utilizing metal oxides and low molecular weight polyols in the magnetic coating layer have appeared. U.S. Pat. No. 4,020,227 describes a magnetic coating composition comprising iron oxide and a binder comprising a polyether polyurethane resin, a glycol of molecular weight greater than 2000 D, and a polyisocyanate.
The amount of hydroxyl-containing species employed in the above formulations typically corresponds to as much as 15-50% of the binder system and 3-10% of the final coating. Also, present methods of incorporation of ingredients often prevent obtaining desired dispersion characteristics due to the large amount of additives used after dispersion has been achieved. Thus, methods and compositions that reduce the amount of post-milling additives are needed. Such improvements would improve the overall quality, reduce the cost of materials for magnetic coatings and improve the mutual compatibility of the magnetic layer components.
Additionally, present magnetic recording media exhibit less than ideal performance with respect to such critical properties as durability, layer-to-layer adhesion, and environmental tape pack stability. It is likely that improvements in these properties can be obtained by identifying more favorable magnetic coating compositions, in particular, by identifying and utilizing binder components that are more favorable to the aforementioned critical properties in the coating compositions.